An example of a disk drive control system known in the art is illustrated in FIG. 5 along with a portion of the control system of a host computer. In FIG. 5, reference numeral 25 denotes a control circuit system of the disk drive, and reference numeral 21 denotes part of the control system of the host computer. This disk drive control system 25 includes an interface section 4, a read/write control section 5, a head position control section 6, and a spindle motor control section 7. The interface section 4 is connected to the read/write control section 5, the head position control section 6, the spindle motor control section 7, and a disk drive interface section 11 in the host computer 21. In addition, the read/write control section 5, the head position control section 6 and the spindle motor control section 7 are respectively connected to a read/write head 8, a head carriage 9 and a spindle motor 10. An instruction from the host computer 21 is sent to the interface section 4 of the disk drive control system 25 through the disk drive interface section 11 in the host computer 21, so that reading/writing of information is performed. When reading/writing is performed, a disk is rotated by the spindle motor 10. Then, the read/write head 8 is moved to a desired track by head carriage 9 so that the formation and detection of recorded magnetization can be performed by the read/write head 8. These operations are respectively controlled by the spindle motor control section 7, the head position control section 6 and the read/write control section 5. A disk drive having the above-described type of arrangement is disclosed in, for example, "Transistor Technology Special, The whole Technique for Utilizing Hard Disks and SCSI, p. 8 (CQ Publishing Company).
Once information is recorded in a conventional disk drive, the information is not copied as long as a person does not give the disk drive a specific instruction to copy. This is because magnetic recording has been considered to be a recording method that is capable of semi-permanently holding information recorded once, in the form of recorded magnetization.
However, if magnetic particles are reduced in size or a medium is made from a thin film in order to increase the recording density of magnetic recording, the recorded magnetization of each bit will weaken with time, due to influences such as from thermal fluctuation. It has been discovered that this phenomenon is particularly serious at high temperatures. This is disclosed in the Journal of Applied Physics 75 (10) (May 1994), IEEE Transactions on Magnetics, Vol. 30, No. 6, (1994), and the Technical Report of the Institute of Electronics, Information and Communication Engineers of Japan, MR94-105, pp. 25-30 (March, 1995)" and so on.
In IEEE Transactions on Magnetics, Vol. 32, No. 1, (January, 1996), it is reported that in the case of a medium using a soft magnetic under layer, the reproduced output may decrease with time. A decrease in recorded magnetization leads to the problem that the reliability of the storage device is impaired because information cannot be correctly reproduced.
The process of conducting a self-diagnosis and predicting trouble (SMART) has been proposed as a means for maintaining or improving the reliability of a disk drive. According to this process, a warning is issued to a host if the number of seek errors or retries during normal use exceeds a reference value. This process is disclosed in DATAQUEST ALERT Apr. 17, 1995). However, in this method, since no warning is issued until an abnormality occurs during normal use, there is a risk that it will be too late to securely restore data when recorded magnetization of the medium has become small.